1. Field of the Invention
The invention refers to a heart stimulator for stimulating at least one atrium and one ventricle of a heart by means of electrical stimulation pulses in an overdrive mode of pacing wherein the atrium and the ventricle are stimulated with an overdrive stimulation rate that is thought to be higher than an intrinsic heart rate. More particular, the invention is directed to dual-chamber (RA-RV), three-chamber (BiA-RV, or RA-BiV), or four-chamber (BiA-BiV) implantable cardiac devices including pacemakers, defibrillators and cardioverters, which stimulate cardiac tissue electrically to control the patient's heart rhythm.
2. Description of the Related Art
Implantable heart stimulators can be used for treating a variety of heart disorders like bradycardia, tachycardia or fibrillation.
Depending on the disorder to be treated, such heart stimulator generates electrical stimulation pulses that are delivered to the heart tissue (myocardium) of a respective heart chamber according to an adequate timing regime. Delivery of stimulation pulses to the myocardium is usually achieved by means of an electrode lead that is electrically connected to a stimulation pulse generator inside a heart stimulator's housing and that carries a stimulation electrode in the region of it's distal end. A stimulation pulse having strong enough strength causes an excitation of the myocardium that in turn is followed by a contraction of the respective heart chamber. A stimulation pulse also is called a pace. Similarly, pacing a heart chamber means stimulating a heart chamber by delivery of a stimulation pulse (pace).
In order to be able to sense a contraction a heart chamber that naturally occurs without artificial stimulation and that is called intrinsic, the heart stimulator usually comprises at least one sensing stage that is connected to a sensing electrode on said electrode placed in the heart chamber. An intrinsic excitation of a heart chamber results in characteristic electrical potentials that can be picked up via the sensing electrode and that can be evaluated by the sensing stage in order to determine whether an intrinsic excitation—called: intrinsic event—has occurred.
Usually, a heart stimulator features separate stimulation generators for each heart chamber to be stimulated. Therefore, in a dual chamber pacemaker, usually an atrial and a ventricular stimulation pulse generator for generating atrial and ventricular stimulation pulses are provided. Delivery of an atrial or a ventricular stimulation pulse causing an artificial excitation of the atrium or the ventricle, respectively, is called an atrial stimulation event AP (atrial paced event) or a ventricular stimulation event VP (ventricular paced event), respectively.
Similarly, common heart stimulators feature separate sensing stages for each heart chamber to be of interest. In a dual chamber pacemaker usually two separate sensing stages, an atrial sensing stage and a ventricular sensing stage, are provided that are capable to detect intrinsic atrial events AS (atrial sensed event) or intrinsic ventricular events VS (ventricular sensed event), respectively.
By means of a sensing stage for a heart chamber to be stimulated, the pacemaker is able to only trigger stimulation pulses when needed that is when no intrinsic excitation of the heart chamber occurs in time. Such mode of pacing a heart chamber is called demand mode. In the demand mode the pacemaker schedules an atrial or a ventricular escape interval that causes triggering of an atrial or ventricular stimulation pulse when the escape interval times out. Otherwise, if an intrinsic atrial or ventricular event is detected prior to time out of the respective atrial or ventricular escape interval, triggering of the atrial or ventricular stimulation pulse is inhibited.
Depending upon which chambers of heart are stimulated and which sense events are used different modes of stimulation become available. These modes of stimulation are commonly identified by a three letter code wherein the first letter identifies the chamber or chambers to be stimulated such as V for a ventricle to be stimulated, A for an atrium to be stimulated and D (dual) for both, ventricle and atrium to be stimulated. Similarly, the second letter characterizes the chamber or chambers sensed events may origin from (V: ventricle, A: atrium, D: ventricle and atrium). The third letter characterizes the mode of delivery of stimulation pulses: T=triggered, I=inhibited and D=dual (T+I). A fourth letter “R” may characterize a rate adaptive heart stimulator that comprises an activity sensor or some other means for determining the hemodynamic need of a patient in order to adapt the stimulation rate accordingly.
A dual chamber pacemaker featuring an atrial and a ventricular sensing stage and an atrial and a ventricular stimulation pulse generator can be operated in a number of stimulation modes like VVI, wherein atrial sense events are ignored and no atrial stimulation pulses are generated, but only ventricular stimulation pulses are delivered in a demand mode, AAI, wherein ventricular sense events are ignored and no ventricular stimulation pulses are generated, but only atrial stimulation pulses are delivered in a demand mode, or DDD, wherein both, atrial and ventricular stimulation pulses are delivered in a demand mode. In such DDD mode of pacing, ventricular stimulation pulses can be generated in synchrony with sensed intrinsic atrial events and thus in synchrony with an intrinsic atrial rate, wherein a ventricular stimulation pulse is scheduled to follow an intrinsic atrial contraction after an appropriate atrioventricular delay (AV-delay; AVD), thereby maintaining the hemodynamic benefit of atrioventricular synchrony.
In some cases, a DDI mode of stimulation may be adequate. In such DDI mode, a ventricular stimulation pulse is not synchronized with a preceding atrial sense event (not “triggered” by an atrial sense event). However, both, atrium and ventricle, are stimulated in a demand mode wherein stimulation pulses are inhibited if an intrinsic event is sensed prior to time out of a respective escape interval.
In particular if an overdrive stimulation is needed, DDI mode pacing may be adequate. When stimulating a heart with an overdrive stimulation rate it is attempted to deliver a (premature) stimulation pulse prior to a possible intrinsic excitation and thus render a respective heart chamber refractory so that it is not susceptible to any further (natural) excitation during a (natural) refractory period needed by the cells of the myocardium to repolarize and thus become susceptible to further excitation again.
Atrial overdrive pacing is useful in a number of applications.
One typical application is to prevent atrial fibrillation (AF). Possible mechanisms by which atrial pacing may be effective include suppression of premature supraventricular beats, elimination of delayed atrial conduction, and atrial pauses that may trigger or facilitate reentry circuits favoring the initiation of AF. Various algorithms have been developed, including dynamic (permanent) atrial overdrive pacing, post-AES (temporary) atrial rate stabilization, post-mode-switch (temporary) overdrive pacing, etc.
Another application is for atrial capture verification during atrial pacing threshold measurement. For patients with intact AV node, the presence of a conducted ventricular sense (Vs) after a premature atrial stimulation pulse (Ap) indicates that the atrial stimulation pulse was strong enough to be effective and thus to cause “capture” of the atrium whereas the absence of a ventricular sense event Vs after the atrial stimulation pulse Ap indicates atrial non-capture. For patient without intact AV node, the atrial non-capture can be suspected on the detection of intrinsic atrial sense (As) after the atrial stimulation pulse Ap since the atrial stimulation pulse was unable to render the atrial myocardium refractory and thus to suppress intrinsic atrial excitation. For both scenarios, atrial overdrive pacing above the intrinsic atrial rate is required.
In a dual-chamber device, atrial overdrive pacing can be achieved in both DDD(R) mode and DDI(R) mode. The DDD(R) mode is useful to maintain the AV synchrony during atrial overdrive pacing, but has intrinsic risk of pacemaker-mediated tachycardia (PMT). On the other hand, the DDI(R) mode is free of PMT but may also lose the hemodynamic benefit of AV synchrony.
Therefore, there is a need to implement the atrial overdrive pacing in DDI(R) mode (thus eliminate the risk of PMT) while still maintaining the AV synchrony (thus enjoy the associated hemodynamic benefits).
For the purpose of this disclosure, the following abbreviations are used:
TABLE 1AbbreviationMeaningAESAtrial extrasystoleApAtrial pace eventAsAtrial sense eventArsAtrial refractory senseAAny atrial eventAsVIThe interval measured from the As to the following Vp orVsAUIAtrial upper intervalAVDAV delay as applied by the pacemaker (in contrast tointrinsic AV delay)FFPWFar field protection window after Vs or VpMSMode switchODIOverdrive pacing interval (s): ODI = 60/OSROSROverdrive stimulation rate (ppm): OSR = 60/ODIPMTPacemaker mediated tachycardiaPVABPost-ventricular atrial blanking periodPVARPPost-ventricular atrial refractory periodRe-SyncRe-synchronization pacing cycle in DDI(R)+ modeSWSafety windowURLUpper rate limitVAIVA interval (duration of the VA timer)VESVentricular extra-systoleVpVentricular paceVsVentricular senseVAny ventricular eventVTVentricular tachycardia